Electron probe microanalyzers and electron microscopes having an attached x-ray spectrometer are used to determine the composition of microscopic or nanoscopic regions of a surface. The detectors determine the energy or wavelengths of x-rays emitted from the sample and infer the composition of material under the electron beam from the energy or wavelength of the x-rays. Because the x-rays characteristic of different materials may have energies that are only slightly different, a detector needs sufficient resolution to differentiate between closely spaced x-ray energies. To process a sample in reasonable amount of time, x-ray detectors need to be able to process a large number of x-rays each second. The number of x-rays that a detector can process each second is referred to as its “maximum count rate”. The rate at which the received x-rays are processed is referred to as the “count rate” and is typically expressed in units of counts per second (cps).
Detectors that use a crystal to disperse and analyze x-rays of different wavelengths are referred to as wavelength dispersive spectrometers (WDS) and detectors that measure the energy of incoming x-rays are referred to as energy dispersive spectrometers (EDS). While a WDS can provide better spectral resolution and greater maximum count rate for a particular wavelength band of x-rays, an EDS is better adapted to measuring x-rays of different energies from multiple elements. Specifically, an EDS can acquire an entire spectrum in parallel, while a WDS is limited to serial acquisition. If two x-rays are received at nearly the same time in an EDS system, the energy measured by the detector will be the result of the both x-rays, and not accurately represent the sample material. Such an event is referred to as “pulse pileup.” EDS detectors preferably process each x-ray quickly, so that each energy measurement is complete before the next x-ray is received.
The most common type of energy dispersive x-ray spectrometer uses a semiconductor x-ray detector in which the x-rays form electron-hole pairs. The electron-hole pairs are detected as an electric current and the number of pairs created by each x-ray depends on the energy of the x-ray. Although EDS systems with solid state detectors typically have a high count rate, up to hundreds of thousands of counts per second, their energy resolution at approximately 6 keV is worse than about 100 eV, which makes it impossible to differentiate closely spaced x-ray peaks.
Another type of energy dispersive x-ray spectrometer is the microcalorimeter-type EDS system, in which an x-ray is absorbed by a detector and the x-ray energy is determined by measuring an increase in temperature of the detector, the increase being proportional to the energy of the absorbed x-ray. The energy resolution of the microcalorimeter detector is superior to that of the semiconductor detector, less than 5 eV at an x-ray energy of approximately 6 keV in some systems, but microcalorimeter detectors typically are only capable of processing less than 500 x-rays per second. Microcalorimeter-type EDS systems are described, for example, in U.S. Pat. No. 5,880,467 to Martinis, et al. for “Microcalorimeter x-ray detectors with x-ray lens” and in Norrell and Anderson, “High Resolution X-Ray Spectroscopy with a Microcalorimeter,” U.S. Department of Energy Journal of Undergraduate Research, Vol. 5, http://www.scied.science.doe.gov/scied/JUR_v5/default.htm (2005).
FIG. 1A shows at typical microcalorimeter-type EDS system 100, which includes a scanning electron microscope 102 and an x-ray optic 104 that transmit x-rays emitted from a sample 106 to a detector 108 cooled by a cryostat 110. X-ray optics are typically either made from glass capillaries or from a thin metallic film, and are described, for example, in U.S. Pat. No. 6,094,471 to Silver et al. for “X-ray Diagnostic System,” and U.S. Pat. No. 6,479,818 to McCarthy et al. for “Application of x-ray optics to energy dispersive spectroscopy.” FIG. 1B shows an enlarged cross section of X-ray optic 104 of FIG. 1. X-ray optics used with a typical prior art microcalorimeter-type EDS has an acceptance angle of two to three degrees.
FIG. 2 shows that detector 108 typically comprises an x-ray absorber 202 and a temperature measuring device 204 in contact with the absorber. The x-ray absorber 202 and temperature measuring device 204 are maintained at a very low temperature, typically below 100 mK, have a very low combined heat capacity and a weak thermal link to a low temperature heat sink 206. The weak thermal link enables the thermal isolation needed for a temperature rise to occur. The output peak height (measured by the temperature measuring device) is related to the x-ray photon energy (E) & the combined heat capacity (C) of the absorber and the temperature measuring device. The energy resolution of the detector is approximately proportional to (kT2C)0.5 (where k is the Boltzmann constant and T is temperature). If the thermal link between the absorber and the low temperature heat sink is made weaker, the temperature of the absorber will rise further, increasing resolution. The weaker thermal link, however, increases the time required to cool the absorber after the x-ray is processed, thereby reducing the maximum count rate that can be processed by the detector.
The x-ray absorbing material is typically gold, and the temperature measuring device employed by most commercial systems includes a transition edge sensor, which includes a layer of non-superconducting material and a layer of superconducting material maintained near its transition temperature, that is, the temperature at which it stops superconducting. An electrical current through the transition edge sensor changes as the temperature of the sensor changes. The change in electrical current is typically amplified using a superconducting quantum interference device (SQUID).
The main technical advantage of microcalorimetry over solid state detectors is superior energy resolution. In the energy range of interest in typical microanalysis, prior art microcalorimeters have a resolution better than 15 eV, and in some cases better than 3 eV, whereas conventional EDS detectors are limited to a resolution of about 120 eV. Hence, microcalorimeters can resolve closely spaced characteristic x-ray lines. This is highly desirable for low voltage microanalysis, that is, microanalysis performed using an electron beam energy in the range of 1-5 keV, because:                The low energy end of the x-ray spectrum contains a large number of closely spaced characteristic x-ray peaks; specifically, the K, L and M lines of low, medium and high atomic number elements, respectively. For many materials, the low energy x-ray peaks overlap in conventional EDS spectra, necessitating the use of higher energy x-ray peaks which can only be excited by high energy (10-30 keV) electron beams.        In scanning electron microscopy, the electron penetration range and the electron-solid interaction volume are approximately proportional to Eb1.67 and (Eb1.67)3, respectively, where Eb is the electron beam energy. X-rays are emitted from some fraction, typically the top one to two thirds of the interaction volume, the exact fraction being a function of the material type and the energy of the x-ray photons and the electron beam. Hence, the surface sensitivity and spatial resolution of microanalysis are strong functions of electron beam energy. Low beam energies are needed for maximum spatial resolution and surface sensitivity.        
The main technical disadvantage of microcalorimetry over conventional EDS is low throughput, caused by two distinct phenomena. First, the solid angle over which x-rays are collected is severely limited by detector design requirements, causing the fraction of emitted x-rays collected by the detector to be very small. Specifically, the surface area of a typical x-ray absorber is on the order of 0.1 mm2 and detector placement close to the sample is inhibited by the bulky nature of the hardware needed to cool the detector to below 100 mK. In contrast, solid state EDS detectors have surface areas in the range of 10 to 80 mm2, and the detectors can be placed within a few centimeters of the sample. The second phenomenon that limits the throughput of microcalorimeter x-ray detectors is that the maximum count rate of a single detector is thermodynamically limited to less than approximately 500 cps. In contrast, the maximum count rates of conventional, solid state EDS (Si(Li) and silicon drift) detectors are on the order of 104 to 105 cps.
The low throughput of microcalorimeter-type EDS systems would require the use of a very high electron beam current to collect sufficient x-rays to form a useful x-ray map in a reasonable time period. A high electron beam current, however impedes electron beam focusing (due to the dependence of electron optical aberrations on beam current), causes rapid damage of electron sensitive samples, gives rise to rapid contamination buildup rates, and gives rise to severe charging of electrical insulators.
FIG. 3 shows portions of an x-ray spectrum of the mineral monazite drawn at three different scales. Enlarged graph 303 shows the large number of peaks available to characterize the sample in the low energy range. While the resolution is sufficiently high to differentiate a large number of closely spaced peaks, it took more than 11 hours to collect enough x-rays to analyze a single point on the sample.
For many applications, it is desired to create high resolution, two-dimensional or three-dimensional x-ray maps of the materials comprising a sample. That is, a region of a sample surface is divided into closely spaced points, and the material present in each point is determined by x-ray analysis, with points mapping to pixels on a display. This has not been possible using prior art microcalorimeter-type EDS systems; the design tradeoffs between high spatial resolution, high energy resolution, and high throughput, have prevented current microcalorimeter-type EDS systems from producing high resolution two or three-dimensional maps in a reasonable amount of time. For example, because the fraction of emitted x-rays that reach the detector is low, a high current electron beam is required to produce more x-rays. Increased beam current increases the size of the electron beam, reducing the spatial resolution of the material analysis at low electron beam energies. Moreover, the count rate of the microcalorimeter detector is limited. When the thermal path between the absorber and the cold substrate is sufficiently weak to provide a high amplitude temperature pulse upon absorption of an x-ray, the absorber takes longer to cool back down after a pulse is detected, reducing the count rate. A more thermally conductive path between the x-ray absorber and the cold sink substrate would allow more pulses per second to be counted, but would reduce the temperature change, and therefore the measurement accuracy of the x-ray energy.
The industry needs an EDS system capable x-ray mapping at high spatial resolution and high energy resolution.